Just as with acid strength, base strength can be determined by considering an equilibrium: The equilibrium constant for a base is Kb. As before, the term for water does not appear in the expression because it is in vast excess, and can be assumed to be constant. It is therefore incorporated into the term for Kb. However, in … Read more
We know from previous discussions that Ka is not an absolute value for a particular substance, because it depends intimately on the surroundings; such as solvent. Most values are quoted for water at 25ºC. It is necessary to quote a temperature with a pKa or Ka, because just as with solvent, Ka varies with temperature. This is, of course, obvious … Read more
A carboxyl group is inductively electron withdrawing. As we have already shown, the presence of electron withdrawing groups can be expected to increase the acidity of compounds. Hence we might expect that compounds with two acid groups would be stronger acids, and this is indeed the case as illustrated by the following values: Carboxylic Acid pKa Dicarboxylic derivative … Read more
The effect of substituents on phenols is not as straightforward as with aliphatic acids. Phenol itself is weakly acidic, with a pKa of 9.95. Above is shown phenol (left), and the phenolate ion (right). The strength of phenol as an acid can be increased by the addition of electron-withdrawing groups. For example, we can add a nitro group (a powerful electron withdrawer). This has both inductive (through σ-bonds) … Read more
This discussion will focus on factors that are internal to the acid which affect its strength. Consider the following acids: Acid Name pKa A Chloroacetic 2.85 B Dichloroacetic 1.25 C Trichloroacetic 0.66 The strength of acid increases with the addition of extra chlorine atoms. This simply illustrates that as X– becomes more electronegative, the anion becomes a more stable conjugate base as it … Read more
Water is the most common solvent with respect to acids and bases. Often, the critical factor in determining the strength of an acid is played by the solvent, especially when that solvent is water. Unfortunately, given that water is an ionising solvent, many organic acids do not dissolve sufficiently in it to begin with. However, if that … Read more
One critical factor in determining the strength of acids can be the strength of the HA bond. Clearly if the proton is very tightly held by the rest of the molecule, it will require a lot of energy to remove it, and hence the acid will be weak. Equally, a weakly-held proton is a characteristic of a strong … Read more
Organic acids have a wide range of strengths. The following discussion will explain some of the trends observed, by reference to Brønsted acids. For example, consider the difference between ethanol and ethanoic acid. The two molecules both have an acidic proton (H+) attached to a heteroatom (in this case, oxygen): Deprotonation of ethanol: Deprotonation of ethanoic acid: So why does ethanoic acid … Read more
The strength of an acid can be determined by considering the extent to which it dissociates in a given solvent (i.e. to which side this equilibrium lies): The equilibrium constant in water is given by: Note that the H2O term has been incorporated into Ka, as water (the solvent) is in vast excess, and hence the value of its concentration will … Read more
There are several definitions of acids and bases. The two most useful and most commonly encountered are the Lewis and Brønsted definitions. According to Brønsted, an acid is a species that has a tendency to lose a proton, and a base is a species with a tendency to gain a proton. According to Lewis, an acid is a species … Read more